Limit switch means, particularly for hydraulic steering circuits

ABSTRACT

The invention relates to limit switch means comprising a sensing element actuated when an operating element reaches an end position, and a switching element which stops the supply of energy to the operating element in response to the sensing element, particularly for hydraulic steering circuits.

United States Patent [191 Olsen Nov. 18, 1975 1 LIMIT SWITCH MEANS, PARTICULARLY FOR HYDRAULIC STEERING CIRCUITS [75] Inventor: Carl Ivar Olsen, Sonderborg,

Denmark [73] Assignee: Danfoss A/S, Nordborg, Denmark [22] Filed: Aug. 5, 1974 [21] Appl. No.: 494,719

Related US. Application Data [63] Continuation of Ser. No. 416,178. Nov. 15, 1973,

abandoned.

[30] Foreign Application Priority Data Nov. 22, 1972 Germany 2257217 [52] US. Cl 251/57; 91/400 [51] Int. CI. F16K 31/122; F1513 15/22 [58] Field of Search 251/23, 57; 91/400 Primary Examiner-Paul E. Maslousky [57] ABSTRACT The invention relates to limit switch means comprising a sensing element actuated when an operating element reaches an end position, and a switching element which stops the supply of energy to the operating element in response to the sensing element, particularly for hydraulic steering circuits.

1 Claim, 3 Drawing Figures 1 9 l/Z 2g 14 2a 29f U.S.-Patent Nov. 18, 1975 3,920,217

b 5 A F/W max LIMIT SWITCH MEANS, PARTICULARLY FOR HYDRAULIC STEERING CIRCUITS This is a continuation of application Ser. No. 416,178, filed Nov. 15, 1973, now abandoned.

In known limit switch means (German Pat. No. 1,124,776), a sensing tappet is provided which is axially displaceable by a frame coupled to a hydraulic operating motor. A switching sleeve in the form of a valve slide is concentrically disposed around the sensing tappet. When the sensing tappet has been displaced by about 1 mm, a hydraulic path is established for placing one end of the switching sleeve under pressure and displacing same to a locking position. This interrupts the supply of pressure medium to the hydraulic motor, which comes to rest in the end position defined by the limit switch means. There is therefore no danger of forcing the machine parts, tools or the like that are operated by the motor against any mechanical abutment or of damaging them in any other way.

However, difficulties are encountered with operating elements which are displaceable at different speeds and move comparatively large masses, as would be the case for example with a hydraulic steering circuit where the speed of the operating motor is proportional to that at which the steering wheel is turned. By reason of the insensing element and this pressure is larger the higher is the speed of the sensing element- The force exerted on the switching element by this pressure is superimposed on the force of the compression spring which increases with an increase in the degree of compression. As soon as the total force exceeds the force of the supporting spring, the switching element will be displaced. Since a greater degree of compression of the compression spring is necessary for lower speeds than is for higher speeds, the desired retardation will be obtained on actuation of the coupling. When the operating element is moved out of the end position, the sensing and switching elements return to their rest position under the ,in-

ertia of the moved masses, high pressures occur at high speeds when there is sudden switching off, and these pressures could reach an impermissibly high value. The pressure could be limited by an additional over-pressure valve but this would mean that the operating motor continues to move further than is desired.

The object of the invention is therefore to provide limit switch means of the aforementioned kind in which the departure of the actual end position that is reached from the desired end position is small despite different speeds of the operating element, and in which the peak pressure in the system will not exceed a predetermined value.

This object is fulfilled according to the invention in that the sensing element is spaced from the desired end position by a distance corresponding to at least the stopping distance at the maximum speed of the operating element and is connected to the switching element by a coupling which retards the response of the switching element with a reduction in the speed of the operating element.

This construction will ensure that the operating element will reach the desired end position even at maximum speed. Although the sensing element will now be actuated prematurely at low speeds, the coupling working with retardation will ensure that substantially the desired end position is reached despite the shorter stopping path.

In a preferred embodiment, the sensing element can be actuated at a speed proportional to that of the operating element, the switching element is displaceable against the force of a supporting spring, and the coupling comprises a compression spring between the sensing and switching elements, the compression spring being weaker than the supporting spring in the rest condition, and a fluid-filled chamber which has a variable volume and has an outlet conduit with a throttle member. By means of the sensing element, the chamber is compressed at a speed proportional to that of the operating element. As a result, the enclosed fluid seeks to escape through the throttle member. This builds up a pressure in the chamber depending on the speed of the fluence of the loaded springs.

In a very simple construction, a sensing tappet, the chamber with the compression spring, a switching tappet and the supporting spring are arranged axially behind one another in a bore. This construction saves space and is also cheap.

A simple throttle passage will suffice as the throttle member. 'One may, however, also provide a throttle member which is adjustable from the outside to permit adaptation to various operating and constructional conditions.

Preferably, the fluid is a hydraulic liquid and the outlet conduit is connected to a nipple for connection to a tank. Since hydraulic liquid is in any case available for a hydraulically-actuated operating motor, this will not cause substantial additional expense. When the sensing and switching elements return to their rest positions, the chamber will be filled with liquid sucked out of the tank. For this purpose it is recommended that the throttle member be bridged by a relief valve so that the suction process is not impeded by the throttle resistance.

It is immaterial if the portion of the bore containing the supporting spring also becomes filled with hydraulic liquid. This portion can then simply be connected to the tank connection nipple by a passage.

In a further embodiment of the invention, the switching tappet can be constituted by a valve slide controlling the hydraulic portion of the operating element.

In a different embodiment, the switching tappet has an end projecting from the bore and actuates an electric contact.

The invention will now be described with reference to an example illustrated in the drawing. In the drawmg:

FIG. 1 is a circuit diagram of limit switch means according to the invention;

FIG. 2 is a distance/speed graph, and

FIG. 3 is a diagrammatic longitudinal section through an embodiment of the invention.

In FIG. 1, a pump 1 delivers oil from a tank 2 through a relief valve 3 to a control appliance 4 which is adjustable by a steering wheel 5. In the rest position, the pressurised oil returns to the tank through a neutral position path 6. An over-pressure valve 7 ensures that the pressure in the control appliance 4 will not exceed a predetermined limiting pressure. Depending on rotation of the steering wheel 5, the compressed fluid can be fed through a conduit 8 or 9 to an operating element 10, for example in the form of an axial motor. The piston ll of the motor has a piston rod 12 which, for example, adjusts the wheels of a vehicle that is to be steered. The piston rod 12 or some other construe? tional part connected thereto contacts a sensing element 13 of limit switch means 14 a considerable distance before reaching the desired end position and, de-

3 pending on the speed of the operating element 10, the limit switch means will react either immediately or after some delay, as hereinafter described. Upon response, the 'limit switch means open a short-circuiting path 15, 16 that surrounds the control appliance 4 and the operating element 10, so that no compressed fluid will any longer be supplied to the operating element 10.

Similar limit switch'm'cans are also provided for the op posite direction of movement, but these have been omitted from the drawing for the sake of clarity.

FIG. 2 shows a graph of the response characteristic P in relation to the speed v of the piston 11 for limit switch means according to the invention. Along the path s of the piston, the desired end position is shown at a and this lies just prior to a mechanical abutment b, for example the upper end of the piston 11. The point c indicates the moment that the piston rod 12 makes contact with the sensing element 13. The difference a c corresponds to the stopping path wnm'r of the moved components at maximum speed. At low speeds the stopping distance w is also smaller. Consequently, retardation takes place between contact of the sensing element 13 at the point c and response of the limit switch means at a point on the line P at speeds less than the maximum.

In the embodiment of FIG. 3, a housing 17 is provided with two end covers 18 and 19 and a longitudinal order to overcome'the force of the supporting spring 24, This corresponds to any point on the line P in FIG. I

2 graph. 'If the sensing tappet 13 is moved at maximum speed, practically no liquid can flow off through the v throttlemember 32. The switching tappet 23 is therebore 20. In this longitudinal bore a sensing tappet 13, a

compression spring 21 in a chamber 22, a switching tappet 23 and a supporting spring 24 in a chamber 25 are provided axially behind one another. The switching tappet 23 has an annular groove 26 which, upon displacement towards the righthand side, interconnects two annular grooves 27 and 28 in the housing 17. The annular groove 27 is connected to a connecting nipple 29 for the short-circuiting conduit and the annular groove 28 leads to a nipple 30 to which the short-circuiting conduit 16 is connected. An outlet conduit 31 leads from the chamber 22 and an externally adjustable throttle member 32 is included in the conduit 31 which leads to a connecting nipple 33 for connection to the tank 2. A relief valve 34 bridges the throttle member 32. A passage 35 connects the chamber to the tank connection nipple 33.

When the sensing tappet 13 is pressed very slowly into the chamber 22, the compressed liquid can flow to the tank 2 through the throttle member 32. It is only when the compression spring 21 has been sufficiently compressed to overcome the force of the supporting spring 24 that the switching tappet 23 becomes displaced to the righthand side to open the short-circuiting path l5, 16. This corresponds to response of the switching tappet 23 at the point a of the FIG. 2 graph.

If the sensing tappet 13 is moved more rapidly, the compressed liquid can flow off only partially through the throttle member 32. A pressure is therefore built up in the chamber 22 and this is superimposed on the fore movedby the sensing tappet 13 practically solely I under hydraulic action. The limit switch means will therefore respond substantially at the point c of the.

FIG. 2 graph.

. If the operating element is moved out of the end position in the opposite direction, the supporting spring 24 pushes the switching tappet 23, and the latter pushes the compression spring 21 and the sensing tappet 13, back to the initial position. This causes the chamber 25 to become larger and take pressure medium out of the tank 2 through the conduit 35, whilst the chamber 22 becomes enlarged and takes pressure medium out of the tank through the relief valve 34 and the throttle member 32.

In the FIG. 3 embodiment, the switching tapper 23 is in the form of a valve slide. It can, however, be extending exteriorly of said casing, second piston means 2 in said bore spaced from said first piston means to form a first chamber therebetween, spring means on the side of said second piston means away from said first piston means, second spring means in said first chamberen- I gaging each of said first and second piston means,

means for supplying a fluid to said first chamber, two

short circuiting ports in said casing having fluid communication with said bore, said ports being separated by said second piston means when said sensing rod is in an at rest position, and said second piston means being movable in response to movement of said sensing rod and first piston means through the medium of said fluid in said first chamber and said second spring means to provide fluid communication between said short circuiting ports, said casing having an exhaust port which has fluid communication with the exterior of said casing and is fluidly separated from said short circuiting ports, and a throttling passage between said first ehamher and said exhaust port to control said response of said second piston means to said movement of said first 

1. A limit stop assembly for a hydraulic steering system comprising a casing having a bore, a sensing element including first piston means in said bore and a sensing rod connected to said first piston means and extending exteriorly of said casing, second piston means in said bore spaced from said first piston means to form a first chamber therebetween, spring means on the side of said second piston means away from said first piston means, second spring means in said first chamber engaging each of said first and second piston means, means for supplying a fluid to said first chamber, two short circuiting ports in said casing having fluid communication with said bore, said ports being separated by said second piston means when said sensing rod is in an at rest position, and said second piston means being movable in response to movement of said sensing rod and first piston means through the medium of said fluid in said first chamber and said second spring means to provide fluid communication between said short circuiting ports, said casing having an exhaust port which has fluid communication with the exterior of said casing and is fluidly separated from said short circuiting ports, and a throttling passage between said first chamber and said exhaust port to control said response of said second piston means to said movement of said first piston means. 